1. Field of the Invention
The present invention relates to an optical short pulse generator using an electro-absorption semiconductor optical modulator. The present invention is applied, for example, to a light source of optical fiber communication.
2. Description of Related Art
Technology to generate optical short pulses using an electro-absorption semiconductor optical modulator (EA modulator) has been known.
An EA modulator is an element to modulate the intensity of light using the Franz-Keldysh effect. The Franz-Keldysh effect is an effect of changing the wavelength absorption edge when voltage is applied to a high resistance semiconductor. In an EA modulator, a light absorption quantity increases when reverse voltage is applied. So by applying sinusoidal voltage to an EA modulator, an optical short pulse train can be generated using continuous wave laser beams.
When optical short pulses are generated by an EA modulator, the extinction characteristics is an important element. The extinction characteristics of the EA modulator is determined by the absorption quantity when a forward voltage is applied and by the absorption quantity when a backward voltage is applied. By improving the extinction characteristics, the loss of the optical short pulse train can be decreased and the minimum value of the pulse width, which the EA modulator can generate, can be decreased.
The magnitude of change of the absorption coefficient depends on the inherent characteristics of the semiconductor, the applied electric field, the detuning quantity and other factors. Detuning quantity is the difference between the band gap wavelength of the light absorption layer and the wavelength of light. The light absorption quantity of an EA modulator changes according to the quantity of light confined to a light wave guide in the EA modulator. Therefore, the extinction characteristics of an EA modulator is roughly determined by the device structure and the characteristics of the material of the light absorption layer.
Currently in the optical communication field, it is demanded to use 10 giga-hertz or higher optical short pulses as a light source. In order to generate 10 giga-hertz or higher optical short pulses using an EA modulator, the capacitance of the element must be very small, so the length of the EA modulator must be 200-300 xcexcm or less. Restricting the size of the EA modulator requires restrictions on the structure of the EA modulator. Therefore, it is very difficult to improve the extinction characteristics by improving the structure of an EA modulator.
In addition, it is not easy to improve the extinction characteristics by changing the material of the light absorption layer either, since control becomes complicated.
A known technology to decrease the pulse width of optical short pulses is the technology proposed by Kanaoka and others. This technology was explained in xe2x80x9c40 Gbit/s OTDM signals transmission experimentxe2x80x9d, 1997, The IEICE Communication Society Convention Papers 2, B-10-117, p. 416.
Kanaoka and others decrease the pulse width by compressing the optical short pulse train generated by an EA modulator using dispersion decreasing fiber (DDF). Dispersion decreasing fiber however is expensive. Another shortcoming of dispersion decreasing fiber is that it not flexible during use, which means that the light intensity must be changed according to the wavelength of the optical pulses to be compressed since the dispersion of light is fixed.
It is an object of the present invention to provide an optical short pulse generator having excellent extinction characteristics, that is, an optical short pulse generator which has low loss and can generate an optical short pulse train with a very short pulse width.
To achieve this object, an optical short pulse generator in accordance with the present invention comprises means for generating optical short pulses from incident light injected into a light absorption layer according to the frequency of the control voltage, means for supplying the incident light to the light absorption layer of the generation means, means for applying the control voltage to the generation means, and means for supplying pump light to the light absorption layer of the generation means in order to generate a pile up.
Generating a pile up by irradiating pump light to the light absorption layer of the generation means can improve the extinction characteristics.